The entire disclosure of Japanese Patent Application No. 10-103549 filed on Mar. 31, 1998, including specification, claims, drawings and summary, is incorporated by reference in its entirety.
The present invention relates to a hydraulic shock absorber of a damping force adjustable type to be mounted on a suspension apparatus of a vehicle such as an automobile and the like.
Hydraulic shock absorbers to be mounted on a suspension apparatus of a vehicle such as an automobile and the like includes a hydraulic shock absorber of a damping force adjustable type, which is adapted so as to adjust the damping force to an appropriate extent in order to improve the riding comfort and stability of operation in accordance with the road situation, running status and the like.
A hydraulic shock absorber of a damping force adjustable type generally comprises a cylinder with an oily fluid filled therein, a piston connected to a piston rod and installed slidably in the cylinder so as to divide the inside of the cylinder into two compartments, and a main oily fluid passage and a bypass for communicating with the two compartments at a piston section. The main oily fluid passage is provided with a damping force generating mechanism comprising an orifice and a disc valve and the bypass is provided with a damping force adjusting valve for adjusting a passage area of the oil path.
The damping force adjusting valve is configured in such a fashion that, on the one hand, the damping force is reduced by decreasing the passage resistance to the passage of the oily fluid passing through the two compartments of the cylinder when the bypath is opened and, on the other hand, the damping force is increased by increasing the passage resistance between the two compartments thereof when the bypath is closed. The damping force characteristics can be adjusted appropriately by opening or closing the damping force adjusting valve in the manner as described above.
For the damping force adjusting valve of the type as adjusting the damping force by changing the passage area of the bypath, the damping force characteristics can be changed to a great extent in a low speed region of the piston speed because the damping force depends upon the restricted size of the oily fluid passage. However, the damping force characteristics cannot be greatly changed in a medium-high speed region of the piston speed because the damping force depends upon the opening degree of the damping force generating mechanism (e.g., disc valve, etc.) of the main oily fluid passage.
As disclosed, for example, in Japanese Patent Application Publication (Kokai) No. 62-220,728, a disc valve acting as the damping force generating mechanism of the main oily fluid passage common on the expanding and contracting sides is provided at the back portion thereof with a pressure chamber (a pilot chamber) so that for the pressure chamber to communicates with a cylinder chamber on the upstream side of the disc valve through a fixed orifice and to communicates with a cylinder chamber on the downstream side of the disc valve through a variable orifice, it is a flow rate control valve.
The hydraulic shock absorber of a damping force adjustable type is configured such that the passage area of the communicating passage between the two cylinder chambers in the cylinder can be controlled by opening or closing the variable orifice and the initial pressure for opening the disc valve can be changed by changing the pressure in the pressure chamber due to the loss of the pressure to be caused at the variable orifice. This configuration can adjust the orifice characteristics, in which the damping force is approximately proportional to a square of the piston speed, as well as the valve characteristics, in which the damping force is approximately proportional to the piston speed, thereby extending the scope of adjustment of the damping force characteristics.
Such conventional hydraulic shock absorber of a damping force adjustable type as disclosed in the prior patent publication is configured such that the damping force actually varies with the magnitude of the piston speed because the damping force is adjusted by controlling the flow rate with the variable orifice. Therefore, if a rapid input would be caused to occur due to the thrust of the road or for other reasons, the damping force is also caused to increase rapidly, together with a rise in the piston speed, thereby transmitting the impact to the vehicle body and as a consequence worsening the riding comfort. Moreover, as the variable orifice varies a passage resistance to a great extent due to the viscosity of an oily fluid, the damping force characteristics are adversely affected to a great extent by changes of temperature, thereby making it difficult to achieve stable damping force characteristics.